1. Field of the Invention
The present invention relates generally to thickness standards and, more particulary, to a thickness standard adapted for use with any of a number of known types of thickness gages.
2. Description of the Prior Art
Thickness gages are commonly used to measure the thickness of coatings such as paint, plating, galvanizing and anodic or organic coatings which have been applied to metallic or non-metallic surfaces such as steel, aluminum or graphite. Typically, these gages must be calibrated prior to use in a measuring operation. One method of carrying out such calibration is to place the gage against an uncoated surface and calibrate the gage to zero thickness. Alternatively, there are unmounted or unsupported films as well as films that have been applied to metallic base materials which are available and which are of a designated thickness and which may be placed beneath a gage during a calibration operation so as to provide a standard thickness by which the gage may be calibrated.
Several drawbacks exist with such known types of calibration. For example, it is not possible, absent the presence of some available standard, to accurately calibrate a gage to a desired precision prior to its use in measuring the thickness of a coating on a surface. In addition, it is desirable to actually calibrate the gage at the location in which the thickness of a coating is to be measured so as to insure that the gage is calibrated as accurately as possible and will not suffer from the effects of conditions related to a separate and remote standard surface which are different from the location of testing.
It is important in light of the fact that coating measurement may be in a very wide range of thickness, usually between 0.00050 and 0.01000 inches, that thickness gage calibration be with a thickness standard selected at a value that is close to the range of thickness to be expected in any particular application, in order to ensure most accurate readings from the thickness measuring instrument.
Conventional films which have been employed in the calibration of thickness measuring gages typically are unguarded, unmounted films which may be easily scratched, broken, warped or damaged so as to render any calibration made with the film to be unreliable. The useful life of such films is very short even if the films are properly stored when not in use.
In addition, there are some known thickness standards employing a film mounted on a relatively heavy supporting surface which may be placed at a position remote from the surface upon which a coating is to be measured. Gages may be placed against the film on the support surface and calibrated by the known thickness of the film. A drawback of this type of thickness standard is that since the surface supporting the standard thickness film is not the actual surface which is to receive a coating, the calibration may be inaccurate. For example, where a magnetic gage is employed, the magnetic characteristics of the surface supporting the standard thickness film may differ from the actual surface which is to be coated so that the calibration of the gage made on the film support surface may be inaccurate.
In addition to there being a need for a thickness standard which may be employed on the actual surface to be coated, there is a need for an apparatus which will permit a thickness gage to be employed in the measurement of uncured powdered material coatings on a surface. Presently, there are no devices available which would permit uncured powdered coatings to be measured at the surface to which they are applied. Such a capability would be advantageous since the thickness of the uncured coating is determinative of the thickness of the final cured coating.